You can add groups as followers by default when you create/edit projects using workflow engine rule. 
There is also an application for adding group members as managers https://www.clarizen.com/apps/group-members-additional-managers/
If you download that application and change the fields from New entity to reviewer you should be on the right track


That adds the group, and the default permission is "Viewer" which is what you want for "Reviewers". You can also make them editors, ehich is almost identical to Manager: